Many melts cannot be crystallised using conventional continuous crystallisation plant (cooling rollers, cooling belts) because the crystallisation process takes place too slowly. Such products am often introduced into crystallisation baths or vats where they then crystallise over the course of days or weeks. Crystallisation plant is also known in which granules are produced from so-called two-phase mixtures. In such plant the starting material is enriched with seed crystals in a so-called precrystalliser and the precrystallised melt is then crystallised out on a cooling belt. DE-A-3 209 747 discloses a precrystalliser for producing seed crystals in melts, for inoculating the melt, which is constructed in the form of a horizontal cylindrical heat exchanger. The crystals formed on the cooled outer walls are continuously scraped off by paddles moving over the walls. The paddles are arranged on a rotating shaft which is driven by a motor. The rotating shaft is mounted on both end faces of the horizontal cylindrical heat exchanger. In order to maintain adequate circulation of the melt in the heat exchanger there is additionally provided an external circuit for melt that has already been "inoculated", by means of which a certain meterable portion of the melt that has already been provided with seed crystals is pumped out of the heat exchanger and mixed with the as yet un-precrystallised starting material at the entrance to the heat exchanger.
The precrystalliser described has a very complicated construction. The requirement for two outer be,wings for the horizontal rotating shaft with the paddles and the additional lines and pumps for circulating a portion of melt that has already been "inoculated" means that such apparatus for producing seed crystals in melts is relatively expensive. In addition, such apparatus constructed in the form of a horizontal heat exchanger together with the necessary additional external circuit takes up a relatively large amount of space.